A slot skill on healers

Honestly, the A skill doesn't matter all that much. I typically make my healers tanker with the A skill slot and inherit Fortress Defense or, if unable to access that, HP +5.

Best way to level healer is to heal high level heroes and not try to kill.

Have a great Def guy and your healer to level and a dancer or two. Kill all except 1 and just keep dancing and healing the tank.

On a slot I usually use +5 hp or def.

Fortress Def or Fortress Res are great for non-Wrathful Staff users. HP +5 and Speed +3 are also good options if you can't get the other two.

As for leveling, I personally just put them on my training team and level or SP farm someone else while they just spam heal and try to stay alive.